An antenna is an electrical conductor or array of conductors that radiates (transmits and/or receives) electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned to the same frequency band that the radio system operates in, otherwise reception and/or transmission will be impaired. Small antennas are required for portable wireless communications. With classical antenna structures, a certain physical volume is required to produce a resonant antenna structure at a particular radio frequency and with a particular bandwidth. Thus, traditionally bandwidth and frequency requirements dictated the volume of an antenna.
The bandwidth of an antenna refers to the range of frequencies over which the antenna can operate satisfactorily. It is usually defined by impedance mismatch but it can also be defined by pattern features such as gain, beamwidth, etc. Antenna designers quickly assess the feasibility of an antenna requirement by expressing the required bandwidth as a percentage of the center frequency of the band. Different types of antennas have different bandwidth limitations. Normally, a fairly large volume is required if a large bandwidth is desired. Accordingly, the present invention addresses the needs of small compact antenna with wide bandwidth. The present invention provides a versatile antenna design that resonates at more than one frequency, that is it is multiresonant, and that may be adapted to a variety of packaging configurations.
A magnetic dipole antenna is a loop antenna that radiates electromagnetic waves in response to current circulating through the loop. The antenna contains one or more elements. Elements are the conductive parts of an antenna system that determine the antenna's electromagnetic characteristics. The element of an magnetic dipole antenna is designed so that it resonates at a predetermined frequency as required by the application for which it is being used. The antenna's resonant frequency is dependant on the capactive and inductive properties of the antenna elements. The capacitive and inductive properties of the antenna elements are dictated by the dimensions of the antenna elements and their interelations.
The radiated electromagnetic wave from an antenna is characterized by the complex vector E×H in which E is the electric field and H is the magnetic field. Polarization describes the orientation of the radiated wave's electric field. For maximum performance, polarization must be matched to the orientation of the radiated field to receive the maximum field intensity of the electromagnetic wave. If it is not oriented properly, a portion of the signal is lost, known as polarization loss. Dependent on the antenna type, it is possible to radiate linear, elliptical, and circular signals. In linear polarization the electric field vector lies on a straight line that is either vertical (vertical polarization), horizontal (horizontal polarization) or on a 45 degree angle (slant polarization). If the radiating elements are dipoles, the polarization simply refers to how the elements are oriented or positioned. If the radiating elements are vertical, then the antenna has vertical polarization and if horizontal, it has horizontal polarization. In circular polarization two orthogonal linearly polarized waves of equal amplitude and 90 degrees out of phase are radiated simultaneously.
Magnetic dipole antennas can be designed with more than one antenna element. It is often desirable for an antenna to resonate at more than one frequency. For each desired frequency, an antenna element will be required. Different successive resonances occur at the frequencies f1, f2, fi . . . fn. These peaks correspond to the different electromagnetic modes excited inside the structure. The antenna can be designed so that the frequencies provide the antenna with a wide bandwidth of coverage by utilizing overlapping or nearly overlapping frequencies. However, antennas that have an wider bandwidth than a monoresonant antenna often have a correspondingly increased size. Thus, there is a need in the art for a multiresonant antenna; wherein the individual antenna elements share volume within the antenna structure.